Modelling and analysis of hydrogenated and dilute nitride semiconductors
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University College Cork
Dilute nitride alloys, containing small fractions of nitrogen (N), have recently attracted research interest due to their potential for application in a range of semiconductor optoelectronic devices (e.g. lasers, light emitting diodes and single photon sources). Experiments have revealed that dilute nitride alloys such as GaAs1−xNx, in which a small fraction x of the arsenic (As) atoms in the III-V semiconductor GaAs are replaced by N, exhibit a number of unusual properties. For example, the band gap energy decreases rapidly with increasing N composition x, by up to 150 meV per % N replacing As in the alloy. This provides an electronic band structure condition which is indeed promising for the development of highly efficient and temperature stable semiconductor optoelectronic devices based on GaAs. We develop a fundamental understanding of this unusual class of semiconductor alloys and identify general material properties which are promising for application in light sources such as light emitting diodes and single photon sources. By performing detailed k·p calculations, we investigate the electronic band structure of nitrogen-free and dilute nitride III-V semiconduc tors. We reinforce our theoretical investigations by comparing our calculations to the results of experimental measurements. We first analyse the optical properties of type-I InAs1−xSbx/AlyIn1−yAs quantum wells (QWs) grown on relaxed AlyIn1−yAs metamorphic buffer layers (MBLs) using GaAs substrates, using a theoretical model based on an eight-band k·p Hamiltonian. The theoretical calculations, which are in good agreement with experiment, identify that the observed enhancement in PL intensity with increasing wavelength is associated with the impact of compressive strain on the QW valence band structure. Via a systematic analysis of strain-balanced quantum well structures we predict that growth of narrow (≈ 4-5 nm) strained wells could lead to a further doubling in optical efficiency for devices designed to emit at 3.3 µm. Analysing the properties and performance of strain-balanced structures designed to emit at longer wavelengths, we rec ommend the incorporation of dilute concentrations of nitrogen (N) to achieve emission beyond 4 µm. We confirm the benefits of growth on relaxed AlyIn1−yAs MBLs, with an Al composition y = 12% providing significantly improved band offsets and optical characterisics compared to a MBL with y = 6%. In the next part of the thesis, we investigate the design of type-II GaAsSb/GaAs quantum ring based (QR) intermediate band solar cells. We present an analytical solution of Schr¨odinger’s equation for a cylindrical QR of infinite potential depth to describe the evolution of the QR ground state with QR morphology, and then undertake 8-band k·p calculations for more de tailed analysis. The calculated electronic properties demonstrate several benefits, including (i) large hole ionisation energies, mitigating thermionic emission from the intermediate band, and (ii) electron-hole spatial overlaps exceeding those in conventional GaAs1−xSbx/GaAs quantum dots. Finally, we turn our attention to modelling hydrogenated InGaAsN/GaAs nanostructures for application as single photon sources at telecommunication wavelengths. The longest wavelength emission achieved to date from such structures is at 1.2 µm. By analysing their electronic band structure and comparing with existing literature data for InGaAsN/GaAs QW structures, we identify a range of QW compositions and well widths for which it should be possible to achieve hydrogenated InGaAsN/GaAs nanostructures emitting at 1.31 µm.
Electronic band structure , Light emitting diode , Solar cell , Single photon source
Arkani, R. 2021. Modelling and analysis of hydrogenated and dilute nitride semiconductors. PhD Thesis, University College Cork.